1. Filed of the Invention
The present invention relates to a magnetic memory device capable of writing and reading data (information) by utilizing a magnetoresistive effect in each of a plurality of magnetic memory cells that are disposed in a matrix form, each magnetic memory cell having a plurality of magnetic layers laminated in a state that these magnetic layers are partitioned by a non-magnetic layers. The present invention also relates to a method of reading data in this magnetic memory device.
In recent years, the above-described magnetic memory device has come to attract attention as a nonvolatile high-density memory. Each of the plurality of magnetic memory cells that constitute a main part of the magnetic memory device is generally formed by having a plurality of magnetic layers laminated in a state that the magnetic layers are partitioned by non-magnetic layers. For example, each magnetic memory cell is formed by two ferromagnetic layers made of thin films of ferromagnetic material that are partitioned by a non-magnetic layer. This magnetic memory device has a function of writing data into a memory cell at an operational position (an address) and reading data from this memory cell, like a DRAM (dynamic random access memory) that has a plurality of memory cells. Thus, this magnetic memory device is also called an MRAM (magnetic random access memory).
In more detail, a resistance due to the magnetoresistive effect of the magnetic memory cell is different depending on whether magnetic moments of the two ferromagnetic layers included in the magnetic memory cell are mutually in parallel directions or in anti-parallel directions. Data of either xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is stored in the magnetic memory cell according to the value of the resistance.
The operation of writing data into the magnetic memory cell is executed by first passing a current through a current line (for example, a word line or a bit line) provided near one of the two ferromagnetic layers. Then, an inversion or a non-inversion of a direction of a magnetic moment of this ferromagnetic layer is controlled based on a magnetic field generated by this current. On the other hand, the operation of reading data from the magnetic memory cell is executed by utilizing the fact that the resistance of the magnetic memory cell is smaller when the magnetic moments of the two ferromagnetic layers are mutually in parallel directions than when the magnetic moments of the two ferromagnetic layers are mutually in anti-parallel directions. In other words, the data read operation is executed by first detecting a relative resistance of the magnetic memory cell based on a flow of a very small current (or fine current) in a direction horizontal to the two ferromagnetic layers, or a flow of a very small current in a direction vertical to the two ferromagnetic layers. Next, a result of this detection is amplified by a sense amplifier, and a decision is made on the data xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d.
2. Description of the Related Art
In order to facilitate understanding of problems of a conventional magnetic memory device, the principle of the operation of a magnetic memory device generally used and conventional examples of a magnetic memory device having a layout of a plurality of magnetic memory cells will be explained below, with reference to FIG. 1 to FIG. 3 that will be described later in xe2x80x9cBRIEF DESCRIPTION OF THE DRAWINGSxe2x80x9d.
A schematic diagram showing the principle of the operation of a magnetic memory cell that utilizes a general magnetoresistive effect, is illustrated in FIG. 1. A perspective view showing a structure of a first example of a conventional magnetic memory device is illustrated in FIG. 2. A perspective view showing a structure of a second example of a conventional magnetic memory device is illustrated in FIG. 2. FIG. 2 and FIG. 3 show structures of a main part of the magnetic memory device, respectively.
Portion (a) to portion (c) of FIG. 1 show a sequence of executing a data write operation to a magnetic memory cell 200 having a structure that two ferromagnetic layers made of thin films of a ferromagnetic material are partitioned by a non-magnetic layer. In general, a thin film of ferromagnetic material is manufactured of Permalloy (usually expressed as Ni-Fe/Co) added with a small volume of cobalt, and a non-magnetic layer is manufactured of aluminum oxide (usually expressed as Al2O3). The magnetic memory cell 200 that includes the two ferromagnetic layers and one non-magnetic layer has such a structure that, on a first magnetic layer 201 at a lower-layer portion within the two ferromagnetic layers, a second magnetic layer 202 at an upper-layer portion is laminated via a non-magnetic layer 203.
As shown in portion (d) and portion (e) of FIG. 1, there arises a difference in the magnetoresistive effect due to the magnetic mutual operation between the first magnetic layer 201 and the second magnetic layer 202, depending on whether the magnetic moments of the first magnetic layer 201 and the second magnetic layer 202 are mutually in parallel directions or in anti-parallel directions. As a result, there arises a difference in the resistance of the magnetic memory cell 200. More specifically, when the magnetic moments of the first magnetic layer 201 and the second magnetic layer 202 are mutually in parallel directions ((d) in FIG. 1), this resistance becomes smaller than when the first magnetic layer 201 and the second magnetic layer 202 are mutually in anti-parallel directions ((e) in FIG. 1). Data xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is stored in the magnetic memory cell corresponding to the value of this resistance.
Assume a state that the magnetic moments of the first magnetic layer 201 and the second magnetic layer 202 are mutually in parallel directions. For example, assume a state that the data of xe2x80x9c0xe2x80x9dhas been stored in advance, before data is written into the magnetic memory cell 200, as shown in portion (a) of FIG. 1. The operation of writing data xe2x80x9c1xe2x80x9d into the magnetic memory cell 200 in this state is executed by first passing a current through a current line (for example, a word line or a bit line) 204 provided near the second magnetic layer 202. Then, a direction of the magnetic moment of the second magnetic layer 202 is inverted based on a magnetic field B generated by this current, as shown in portion (b) of FIG. 1. Thereafter, the direction of the magnetic moment of the second magnetic layer 202 maintains the invented state even after the current of the current line 204 has been set to be zero and the magnetic field B has been removed, as shown in portion (c) of FIG. 1. Therefore, the data of xe2x80x9c1xe2x80x9d is stored in the magnetic memory cell. In this case, a component ratio of each element (iron, nickel, cobalt, etc.) of the magnetic material for the first magnetic layer 201 and the second magnetic layer 202 is changed in advance so that the magnetic moment of the second magnetic layer 202 is inverted in a magnetic field smaller than that of the first magnetic layer 201. Based on this arrangement, it becomes easy to invert only the magnetic moment of the upper-layer second magnetic layer 202, without influencing the magnetic moment of the lower-layer first magnetic layer 201.
On the other hand, for executing the operation of reading data stored in the magnetic memory cell 200, a fine current is supplied to the magnetic memory cell 200 from a power source Vd via a current line 204, as shown in portions (d) and (e) of FIG. 1. Based on this current supply, a difference between two resistances is detected as follows. That is, a difference is detected between the resistance of the magnetic memory cell 200 when the magnetic movements of the first magnetic layer 201 and the second magnetic layer 202 are mutually in parallel directions and the resistance of the magnetic memory cell 200 when these magnetic moments are mutually in anti-parallel directions. Based on this, a decision is made as to whether the data in xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d.
In a spin-valve type ferromagnetic memory device 100 as a first conventional example shown in FIG. 2, a data read operation is carried out by passing a current in a direction horizontal to two ferromagnetic layers within the magnetic memory cell 200. In this case, a relative resistance that is generated due to a giant magnetoresistance effect (usually abbreviated to a GMR) of the two ferromagnetic layers is detected. On the other hand, in a tunneling-type ferromagnetic memory device 101 as a second conventional example shown in FIG. 3, a data read operation is carried out by passing a current in a direction vertical to two ferromagnetic layers within the magnetic memory cell 200. In this case, a relative resistance that is generated due to a tunneling magnetoresistive effect (usually abbreviated to a TMR) of the two ferromagnetic layers is detected.
More specifically, a plurality of magnetic memory cells 200 are prepared in the magnetic memory devices of the first and second conventional examples, respectively. These magnetic memory cells are disposed in a matrix form of points where a plurality of word lines (only w0 and w1 are shown here) and a plurality of bit lines (only b0, b1 and b2 are shown here) cross.
In the spin-valve type ferromagnetic memory device 100 as the first conventional example shown in FIG. 2, the magnetic memory cells that are disposed corresponding to the crossings between the word lines w0 and w1 and the bit lines b0, b1 and b2 are defined as MS11, MS12, MS13, MS21, MS22 and MS23, respectively. Current guide portions 205 are formed on both ends of each of these magnetic memory cells MS11 to MS23, respectively. For reading data from a certain magnetic memory cell as a target cell of the data reading, a word line and a bit line located at the corresponding magnetic memory cell are selected respectively. Further, the selected word line and bit line are set to a predetermined potential, and a current I flows in a direction parallel to the ferromagnetic layers within this magnetic memory cell via the current guide portions 205. Further, a sense amplifier or the like amplifies the current that flows out from this magnetic memory cell, and this current is compared with a reference current. Then, a relative resistance of this magnetic memory cell is detected, and the data xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is determined.
On the other hand, in the tunneling-type ferromagnetic memory device 101 as the second conventional example shown in FIG. 3, the magnetic memory cells that are disposed corresponding to the crossings between the word lines w0 and w1 and the bit lines b0, b1 and b2 are defined as MT11, MT12, MT13, MT21, MT22 and MT23, respectively. In this case, the word lines and the bit liens are directly connected to the lower-layer portions and the upper-layer portions of the magnetic memory cells MT11 to MT23, respectively, unlike in the above first conventional example. For reading data from a certain magnetic memory cell as a target cell of the data reading, a word line and a bit line located at the corresponding magnetic memory cell are selected respectively. Further, the selected word line and bit line are set to a predetermined potential, and a current I flows in a direction vertical to the ferromagnetic layers within this magnetic memory cell. Further, a sense amplifier or the like amplifies the current that flows out from this magnetic memory cell, and this current is compared with a reference current. Then, a relative resistance of this magnetic memory cell is detected, and the data xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is determined.
As explained above, according to the conventional magnetic memory device having magnetic memory cells, each in a structure having two ferromagnetic layers laminated together, the data of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is read based on a detection of a current that flows out from the magnetic memory cells.
According to the conventional magnetic memory device, however, each magnetic memory cell can store only two-valve (two-bit) data xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. Therefore, when the provision of a memory having a larger memory density than the conventional memory, or a provision of a multi-functional memory has come to be required along the increase in the capacity of a computer system, it is difficult to meet such a requirement.
Further, according to the above magnetic memory device, a current is passed through the word lines and bit lines for writing data into the magnetic memory cells, and a current flowing out from a selected magnetic memory cell is detected for reading data. Therefore, there is an inconvenience in that the total power consumption of the magnetic memory device becomes relatively large.
The present invention has been made in the light of the above problems. It is, therefore, an object of the invention to provide a magnetic memory device which is capable of realizing magnetic memory cells having a higher recording density than that of the conventional magnetic memory cells, and also capable of realizing multi-functional magnetic memory cells, and also capable of reducing power consumption, and to provide a method of reading data in this magnetic memory device.
In order to achieve the above object, a magnetic memory device of the present invention has a plurality of magnetic memory cells. Each magnetic memory cell is formed to have at least three magnetic layers including a first magnetic layer, a second magnetic layer and a third magnetic layer laminated together. Further, each magnetic memory cell has a resistance that is different depending on directions of magnetic moments of the first, second and third magnetic layers, respectively. The plurality of magnetic memory cells are laid out along crossings between a plurality of first lines and a plurality of second lines that cross the first lines, respectively. The magnetic memory device selectively passes a predetermined current through the first lines and the second lines and controls directions of magnetic moments of the first, second and third magnetic layers, thereby to write data into a specific magnetic memory cells.
In the above structure, each of the first lines consists of at lest two word lines, and each of the word lines individually controls a direction of a magnetic moment of any one of the first, second and third magnetic layers. Thus, data of two or more values is stored in the specific magnetic layer.
Preferably, in the magnetic memory device of the present invention, each of the first lines consists of two word lines. The two words lines are provided at an upper-layer portion and a lower-layer portion of each of the magnetic memory cells, respectively. In this structure, the word line in the upper-layer portion controls a direction of a magnetic moment of a magnetic layer formed on the upper-layer portion, and the word line in the lower-layer portion controls a direction of a magnetic moment of a magnetic layer formed on the lower-layer portion.
More preferably, in the magnetic memory device of the present invention, each of the second lines consists of two bit lines. The two bits lines are provided at an upper-layer portion and a lower-layer portion of each of the magnetic memory cells, respectively. In this structure, the word line in the upper-layer portion and the bit line in the upper-layer portion control a direction of a magnetic movement of a magnetic layer formed on the upper-layer portion. Further, the word lines in the lower-layer portion and the bit lines in the lower-layer portion control a direction of a magnetic moment of a magnetic layer formed on the lower-layer portion. Thus, multi-value data is stored.
More preferably, in the magnetic memory device of the present invention, in carrying out a data read operation by detecting a difference in the resistance of the magnetic memory cell selected by the first line and the second line, the magnetic memory device controls a direction of a magnetic moment of a magnetic layer of each magnetic memory cell in which there is disposed one word line located at a side not having influence on a current flowing through the second lines, out of the two word lines that constitute the first lines. The magnetic memory device controls the direction for each of this word line.
In a method of carrying out a data read operation (i.e. a method of reading data) by detecting a difference in the resistance of the magnetic memory cell selected by the first line and the second line in the magnetic memory device having the above-described structure, a direction of a magnetic moment of a magnetic layer of each magnetic memory cell in which there is disposed one word line located at a side not having influence on a current flowing through the second line, is controlled of reach of this word line. Further, the direction of the magnetic moment of this magnetic layer corresponds to a selection or non-selection of the magnetic memory cell by the first line.
According to the magnetic memory device and the method of reading data described above, a magnetic layer in which one word line is disposed can have a function equivalent to isolating-diodes or isolating-transistors capable of isolating a plurality of magnetic memory cells from each other. Therefore, it is possible to avoid the necessity of providing the isolating-diodes or the isolating-transistors.
Further, a magnetic memory device of the present invention has a plurality of magnetic memory cells. Each magnetic memory cell is formed to have a plurality of magnetic layers laminated together, and each magnetic memory cell has a resistance that is different depending on directions of magnetic moments of the plurality of magnetic layers. In this structure, the plurality of magnetic memory cells are laid out along crossings between a plurality of first lines and a plurality of second lines that cross the first lines, respectively. Further, voltage reading means (or a voltage reading unit) such as a capacitor having a capacity substantially equal to the capacity of each magnetic memory cell is connected in series with each magnetic memory cell. In the above structure, the magnetic memory device measures a potential at a connection point between one terminal of the magnetic memory cell selected by the first line and the second line and the voltage reading means at a predetermined timing. The magnetic memory device decides a voltage level of this potential, thereby reading the data of this magnetic memory cell.
In a method of carrying out a data read operation by detecting a difference in the resistance of the magnetic memory cell selected by the first line and the second line in the magnetic memory device having the above-described structure, voltage reading means having a capacity substantially equal to the capacity of each magnetic memory cell is connected in series with each magnetic memory cell. Further, a potential at a connection point between one end of the magnetic memory cell selected by the first line and the second line and the voltage reading means is measured at a predetermined timing, thereby deciding on this voltage level.
In summary, according to the present invention, in magnetic memory cells, each having a structure of having at least three magnetic layers laminated together, a direction of a magnetic moment of any one of the at least three magnetic layers is controlled individually using at least two word lines. Based on this arrangement, a specific magnetic memory cell stores data of two or more values. Therefore, it is possible to realize magnetic memory cells having a higher recording density that the conventional magnetic memory cells, as well as multi-functional magnetic memory cells.
Further, according to the present invention, voltage reading means such as a capacitor is connected in series with magnetic memory cells, each having a plurality of magnetic layers laminated together. Data of a magnetic memory cell is read by deciding a voltage level obtained by charging this voltage reading means. Therefore, it is possible to reduce power consumption to a lower level than has been required conventionally.